Responses and acclimation to salinity in the adults of some balanomorph barnacles

Abstract

The responses of a number of barnacles to a wide range of salinity have been studied by observation of the activity and measurement of the depression of freezing point of the blood. In active barnacles of the species Elminius modestus, Balanus balanoides, B. crenatus, B. improvisus, B. hameri, B. balanus and Chthamalus stellatus the blood concentration conforms with changes in the external salinity. The concentration of the blood tends to remain slightly hyperosmotic to the fluid in the mantle cavity, and to the medium. With sudden changes of external salinity the blood concentration conforms within a few hours if cirral activity is maintained. When placed in such low salinities that activity is inhibited, E. modestus, B. balanoides, B. crenatus, B. improvisus, B. balanus and C. stellatus close the opercular valves with the result that the blood and mantle cavity fluid are maintained for some time at a level initially considerably hyperosmotic to the medium, but the blood is still only slightly hyperosmotic to the fluid remaining in the mantle cavity. There is no permanent control, and in time the blood concentration approximates to the external level. E. modestus, B. balanoides and B. improvisus from low salinity estuarine habitats, and B. crenatus after gradual reduction of salinity in the laboratory over a matter of days, exhibit tolerance to lower salinities than do specimens of the same species obtained from, or acclimated to normal salinities. Salinity acclimation is typical of osmoconformers lacking specific organs for effective regulation. It is concluded that the barnacles here tested are osmoconformers, able to adjust to small changes of environmental salinity by tissue acclimation, but evading too severe salinity changes by withdrawing into the protection of the shell. The deep sea B. hameri , however, does not close up when immersed in dilute sea water, and appears to be relatively stenohaline with limited ability to acclimate to low salinity. The intertidal E. modestus and B. balanoides , and the low-tidal to sublittoral B. crenatus , are tolerant, after experimental or natural acclimation, of salinities down to 14 to 17 ‰. The estuarine B. improvisus can, with gradual acclimation, be induced to be active in a salinity of about 2 ‰ . This species is remarkably tolerant of dilution of the blood, and its distribution into regions of low salinity is evidently due to a wide tissue resistance and not to any ability to regulate.